1. Field of the Invention
This invention relates to novel compounds containing metals in the +4 oxidation state bonded to four amidinate ligands. This invention also relates to the use of these compounds as precursors for vapor deposition of metal-containing layers.
2. Description of the Related Art
Electrically insulating materials with high dielectric constants (“high-k dielectrics”) are now being used in the manufacture of computer memories (dynamic random access memories, or DRAMs). Aluminum oxide and tantalum oxide are currently in commercial use in DRAMs, and oxides, nitrides and silicates of hafnium and zirconium are being tested as alternatives for future use. These high-k materials may also be used as insulators in future transistors in microelectronic devices.
Electrically conductive nitrides of metals such as tantalum, tungsten, hafnium, zirconium, titanium, niobium and molybdenum have a variety of applications and potential applications, such as barriers against the diffusion of copper, and as electrodes for capacitors and transistors in microelectronic devices. These refractory metals also find use as adhesion-promoting layers for copper, and as electrodes or electrical interconnections.
Vapor deposition is a preferred method for making these materials. Vapor deposition is a generic term that comprises chemical vapor deposition (CVD) and atomic layer deposition (ALD). In a CVD process, one or more vapors are delivered to a surface on which solid material is deposited; the chemical reactions that convert the vapor to a solid are initiated by means such as heat, light or electrical excitation (plasma activation). In an ALD process, two or more vapors are delivered alternately to the surface on which reactions take place to deposit a solid product. ALD is capable of depositing these materials inside the very narrow structures in modern DRAMs. CVD generally provides higher deposition rates than ALD, but with less uniform deposition inside very narrow holes.
Successful precursors for vapor deposition must be volatile, thermally stable, and highly reactive. Identifying compounds that meet all of these challenging requirements is difficult. Fully satisfactory precursors for metals such as hafnium, zirconium, tantalum, niobium, tungsten, molybdenum, tin, tellurium and uranium are not known. Halides, such as ZrCl4, HfCl4, and TaCl5, have difficulty nucleating on some substrate surfaces, and the byproduct hydrochloric acid prevents fully conformal deposition inside narrow holes. Alkoxides and dialkylamides have less than optimal thermal stabilities. Organometallic compounds may lack suitable reactivity, leaving carbon as an impurity in the films. Thus there is a need for more volatile, thermally stable, and highly reactive sources for these metals.